Field of the Invention
The present invention relates to a method for diagnosing the degree of deterioration of a catalyst oxidizing or adsorbing an unburned hydrocarbon gas.
Description of the Background Art
Conventionally, concerning OBD (function diagnosis) of an exhaust gas purification system for automobile, that is, a function of determining whether a catalyst works normally or not in the system, many patent applications are filed centered of automotive companies; however, most of them are techniques using an exhaust gas temperature sensor, an oxygen sensor, a wide range oxygen concentration sensor (λ sensor), a NOx sensor, and a PM sensor, and a diagnosis target is a three-way catalyst, an oxidation catalyst, a NOx storage catalyst, a NOx selective reduction catalyst, and a diesel particulate filter (DPF) (e.g., Japanese Patent Application Laid-Open No. 2001-263048, Japanese Patent Application Laid-Open No. 2005-240716, Japanese Patent Application Laid-Open No. 2012-036860, Japanese Patent Application Laid-Open No. 2012-241594, and Japanese Patent Application Laid-Open No. 07-103039 (1995)).
On the other hand, as a hydrocarbon gas sensor (HC sensor) for automobile exhaust gas measurement, sensors of various principles and various types are researched and developed. For example, a semiconductor type HC sensor which is applicable for purification control of NOx in a zeolite catalyst and can selectively detect HC (hydrocarbon) having a large molecular weight, is already publicly known (e.g., refer to Japanese Patent No. 2876793). As other HC sensors, sensors of a catalytic-combustion type, an oxygen-concentration difference detection type, a limiting current type, a mixed-potential type and the like are widely known, and a large number of patent applications have been filed at the peak of around year 2000.
It is supposed that these HC sensors are mainly used for purification performance inspection of exhaust gas purification devices (TWC: three-way catalyst, NSC: NOx storage catalyst) to be mounted on a gasoline engine of theoretical equivalent ratio (air excess ratio λ=1) combustion or lean burn (lean burn type, λ>1), or the control of injection amount of unburned hydrocarbon in a diesel engine.
Entering the 2010s, exhaust gas regulations are enhanced in North America, and among others, OBD of an oxidation catalyst to diesel engine vehicles will be obliged in the future. Specifically, the need for performing OBD targeted at NMHC (Non Methane HydroCarbon) in O2 (oxygen)-excess atmosphere which is an exhaust gas atmosphere of the diesel engine against an oxidation catalyst for a diesel engine, has arisen.
However, there is a problem that as disclosed in Japanese Patent Application Laid-Open No. 2001-263048, Japanese Patent Application Laid-Open No. 2005-240716, Japanese Patent Application Laid-Open No. 2012-036860, Japanese Patent Application Laid-Open No. 2012-241594 and Japanese Patent Application Laid-Open No. 07-103039 (1995), a techniques using publicly known sensor cannot respond to such OBD or can only indirectly diagnose.
For example, Japanese Patent Application Laid-Open No. 2001-263048 discloses a technique of using a relation that when the capability to convert (oxidize or combust) unburned hydrocarbon in an oxidation catalyst is deteriorated, heat generation energy is also lowered. Schematically describing, in this technique, a temperature difference ΔT generated at the time of fuel injection in exhaust gas temperature sensors arranged upstream and downstream of an oxidation catalyst in an exhaust path is measured, and from the measured values, a degree of deterioration of capability to convert (oxidize or combust) unburned hydrocarbon in the oxidation catalyst is indirectly diagnosed.
However, in such a method, there is a problem that error cause due to changes in an exhaust gas temperature and exhaust gas flow rate in actual use is too large, and a problem that a fuel injection amount for accelerating heat generation is large and therefore deterioration of fuel economy cannot be avoided.
Further, Japanese Patent Application Laid-Open No. 2005-240716 discloses a technique of using the fact that when the capability to convert unburned hydrocarbon in an oxidation catalyst is deteriorated, consumption of oxygen during oxidative combustion is changed. Schematically describing, this is a technique in which a degree of deterioration of capability to convert unburned hydrocarbon in the oxidation catalyst is indirectly diagnosed, based on a difference Δλ between output values λF and λR of two wide-range oxygen concentration sensors (λ sensor) arranged upstream and downstream of an oxidation catalyst in an exhaust path, or from a change in value of oxygen consumption in the oxidation catalyst measured based on a difference between output values (electromotive force values) of two oxygen sensors.
However, the oxygen concentration in the diesel exhaust gas being an O2-excessive atmosphere is about 10% (=100000 ppm), whereas an amount (concentration) of hydrocarbon which the oxidation catalyst converts (oxidizes or combusts) is usually about several hundreds ppm, and an amount (concentration) of oxygen to be consumed in burning such a slight amount of hydrocarbon is also only about several hundreds ppm at most. That is, this means that in order to diagnose the deterioration of an oxidation catalyst by use of the air-fuel ratio sensor or the oxygen sensor, it is necessary to calculate, with accuracy, Δλ or a difference in electromotive force which corresponds to a change in an oxygen consumption of ppm order, but measurement accuracy of such an extent cannot be attained in the first place in the air-fuel ratio sensor and the oxygen sensor.
Japanese Patent Application Laid-Open No. 2012-036860 discloses a technique of determining a deterioration degree of an oxidation catalyst, which oxidizes NO to NO2, based on an output value (electromotive force value) of a NOx sensor arranged downstream of the oxidation catalyst in an exhaust path, and a map previously defined.
However, even though capability to oxidize NO of the oxidation catalyst can be diagnosed by such a technique, the result of such diagnosis cannot be applied to the diagnosis of the capability to convert (oxidize or combust) unburned hydrocarbon. The reason for this is that since functions of a noble metal catalyst and a storage material vary with various gases (for example, HC, CO, NO, etc.), a relation between the exhaust gas temperature and the conversion rate (oxidizing capability) in each gas varies and there is not a definite correlation between them.
In addition, an estimated value is used for NOx value in the exhaust gas immediately after discharge from engine, and factors other than an engine speed and an engine load are not considered in setting such an estimated value, and therefore estimation accuracy may be significantly lowered depending on usage conditions.
Further, Japanese Patent Application Laid-Open No. 2012-241594 discloses a method in which an exhaust gas temperature sensor and a λ sensor are arranged upstream and downstream of an oxidation catalyst, and a deterioration degree of the oxidation catalyst is diagnosed based on a required oxygen amount obtained from an estimated value of HC storage amount at the time of normal state of the oxidation catalyst, and an estimated value of an actual oxygen consumption being an oxygen amount actually consumed by the oxidation catalyst which is computed based on an output value of the sensor.
However, in such a method, the diagnosis is only performed based on estimated values, and there is a problem that an influence of error of a signal from each sensor cannot be avoided and the diagnosis accuracy is low.
Further, Japanese Patent Application Laid-Open No. 07-103039 (1995) discloses an engine system whose target is TWC or NSC of a gasoline engine. Japanese Patent Application Laid-Open No. 07-103039 (1995) does not disclose diagnosis of an oxidation catalyst in the diesel exhaust gas in an O2-excessive state at all.